In most modern fuel cell systems, a compressor provides compressed air to a fuel cell stack, and a water vapor transfer unit humidifies the compressed air before it enters the fuel cell stack. The temperature of the air stream exiting the compressor, at the air mass flow rates required for fuel cell operation, are typically beyond the desirable thermal limit of the water vapor transfer unit.
A control system typically employs a heat exchanger to keep the temperature of the air stream exiting the compressor below the thermal limit of the water vapor transfer unit.
In such fuel cell systems coolant flow to the heat exchanger cannot be stopped. There are situations where it may be desirable to heat the air stream entering the cathode, such as starting the fuel cell, but the coolant entering the heat exchanger is at a lower temperature and cools the air. Therefore, the time required to reach a desired operating temperature is unnecessarily extended. Additionally, such fuel cell systems do not maintain active control of the air stream entering the water vapor transfer unit, and are not capable of maintaining a desired temperature. This prohibits the water vapor transfer unit from performing at an optimum level.
It would be desirable to develop a method and apparatus for accurately controlling the temperature of the air stream entering the water vapor transfer unit, which would maintain a desired temperature and minimize the time required for the air stream to reach a desired operating temperature.